In safety-relevant systems it is important to promptly detect faulty behavior of the sensors used for signal sensing, or of their signal paths. In a motor vehicle this relates, for example, to an acceleration sensor that is used to control airbag systems or vehicle stability systems.
The fault detection mechanisms usually used in the sensor are based substantially on the principle of redundancy or on that of stimulus.
With a redundant measurement, the measured variable is usually sensed using at least one additional sensor whose expected signal can be calculated by a non-trivial linear combination of all other measured signals, the sensing and evaluation of said signal occurring as independently as possible. It is assumed in this context that the sensors are approximately linear and time-invariant. Faulty behavior by one of the sensors being used is then detected on the basis of a significant deviation from the linear correlation for measured signals. The method has the disadvantage, in particular with offset-free or offset-corrected signals, that e.g. gain errors in the signal sensing system can be detected only when a measured variable is present. When the measured variable has a value of zero, a gain error is not detectable from the linear correlation of the redundancy.
Patent document U.S. Pat. No. 7,516,038 B2 presents an example of redundant evaluation of signals of an acceleration sensor. For a horizontally oriented vehicle a gain error of a sensor measuring in the horizontal plane cannot be detected here on the basis of redundancy.
With the stimulus principle, the measured variable furnished by the sensor element is overlaid with a known stimulus signal with which the measurement channel can be checked. The difficulty lies in distinguishing the effects of the stimulus signal and usable signal on the sensed signal, especially if the measurement channel cannot be modeled with sufficient accuracy and if time-related separation is not possible due to continuous sensor sensitivity, and if frequency separation is not possible because of the measurement channel properties.
An example of an additional excitation of an acceleration sensor by an electrical signal may be gathered from DE 10 2010 029903 A1.